BMI Calculator

BMI stands for Body Mass Index, a screening number derived from your height and weight and nothing else. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is widely used in medicine as a quick population-level health indicator. It’s a screening tool, not a diagnosis.

The Formula

Metric: BMI = weight (kg) / height (m)²

Imperial: BMI = (weight (lbs) × 703) / height (inches)²

The 703 is only a unit conversion factor. Both formulas give the same number for the same person.

Worked Example

A person weighs 75 kg and is 1.72 m tall. BMI = 75 / (1.72)² = 75 / 2.9584 = 25.35

A 5'8" (68 inches), 165 lb person: BMI = (165 × 703) / 68² = 115,995 / 4,624 = 25.09

Both fall in the Overweight band (25.0 to 29.9).

WHO BMI Classification (Adults)

Healthy Weight Range for Your Height

The healthy BMI band translates to a weight range:

Lower limit (kg) = 18.5 × height² (m) Upper limit (kg) = 25 × height² (m)

For someone 1.72 m tall:

• Lower healthy weight: 18.5 × 2.9584 = 54.7 kg (121 lb)
• Upper healthy weight: 25 × 2.9584 = 74.0 kg (163 lb)

A note on that 25. The band is conventionally written “18.5 to 24.9”, which makes it look as though 24.95 belongs nowhere. It is really 18.5 up to but not including 25, and the 24.9 is just what you get when you write a half-open interval with one decimal place. This calculator uses 25 as the upper limit for both the category and the weight range, so the two can never disagree with each other.

What BMI Doesn’t Tell You

BMI does not distinguish between fat and muscle. Athletes with high muscle mass often score in the Overweight range despite being very lean. It also says nothing about where fat sits, and abdominal fat carries considerably more health risk than the same weight on hips and thighs.

For a fuller picture, pair BMI with:

• Waist circumference: over 40 inches for men or 35 for women indicates higher risk whatever the BMI says
• Waist-to-height ratio: keep it under 0.5. Increasingly treated as a better single indicator than BMI, and it needs one tape measure
• Body fat percentage: measured with skinfold calipers, a DEXA scan, or (roughly) a smart scale

When BMI Is Less Reliable

• Elderly adults lose muscle and bone mass, so a slightly higher BMI (24 to 27) often correlates with better outcomes
• Children and teens use age-and-sex-specific percentile charts, not adult cutoffs
• Pregnant women should use pre-pregnancy BMI, not current
• Athletes and bodybuilders routinely score “overweight” while having very low body fat
• Asian populations may face elevated health risk at lower BMI thresholds (some health bodies use 23 as the overweight cutoff for Asian adults)

What to Do Next

If your BMI is in the healthy range, the priority is maintenance: regular activity, balanced eating, sleep. If it’s outside the range, the most-evidence-based first step is a small calorie surplus or deficit (300 to 500 per day) combined with strength training, not extreme diets. Big swings tend to reverse; small consistent changes stick.